Pattern diversity to support a MIMO communications system and associated methods

ABSTRACT

A MIMO communications system includes a transmitter, and a receiver synchronized with the transmitter. The transmitter changes the power levels for each layered space data stream on a time slotted basis. The data streams arrive at the receiver with various power levels, which provides suitable differences in the received signals for population of a mixing matrix for signal separation processing.

RELATED APPLICATION

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 11/232,610 filed Sep. 22, 2005 now U.S. Pat. No.7,116,271 which claims the benefit of U.S. Provisional Application Ser.Nos. 60/642,941 filed Jan. 11, 2005; 60/639,223 filed Dec. 23, 2004;60/621,113 filed Oct. 22, 2004; 60/620,775 filed Oct. 20, 2004;60/620,776 filed Oct. 20, 2004; 60/620,862 filed Oct. 20, 2004;60/615,338 filed Oct. 1, 2004; 60/615,260 filed Oct. 1, 2004; 60/612,546filed Sep. 23, 2004; 60/612,435 filed Sep. 23, 2004; 60/612,433 filedSep. 23, 2004; 60/612,550 filed Sep. 23, 2004; 60/612,632 filed Sep. 23,2004; 60/612,548 filed Sep. 23, 2004; 60/612,471 filed Sep. 23, 2004;60/612,551 filed Sep. 23, 2004; 60/612,469 filed Sep. 23, 2004; and60/612,547 filed Sep. 23, 2004 the entire contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of communication systems, andmore particularly, to a multiple-input multiple-output (MIMO)communications receiver operating with a compact antenna array.

BACKGROUND OF THE INVENTION

A multiple-input multiple-output (MIMO) wireless communications systemincludes a plurality of antenna elements at the transmitter and aplurality of antenna elements at the receiver. A respective antennaarray is formed at the transmitter and at the receiver based upon theantenna elements associated therewith.

The antenna elements are used in a multi-path rich environment such thatdue to the presence of various scattering objects in the environment,each signal experiences multipath propagation. The receive antennaelements capture the transmitted signals, and a signal processingtechnique is then applied to separate the transmitted signals andrecover the user data.

The signal processing technique may be a blind source separation (BSS)process. The separation is “blind” because it is often performed withlimited information about the transmit signals, the sources of thetransmit signals, and the effects that the propagation channel has onthe transmit signals. Three commonly used blind signal separationtechniques are principal component analysis (PCA), independent componentanalysis (ICA) and singular value decomposition (SVD).

MIMO communications systems are advantageous in that they enable thecapacity of the wireless link between the transmitter and receiver to beimproved. The multipath rich environment enables multiple orthogonalchannels to be generated therebetween. Data for a single user can thenbe transmitted over the air in parallel over those channels,simultaneously and using the same bandwidth.

Current MIMO communications systems use spatially diverse antennaelements so that the number of orthogonal channels that can be formed isnot reduced. The problem with such an implementation is that theperformance of a MIMO communications system is usually proportional tothe number of antenna elements used.

Increasing the number of antenna elements increases the size of theantenna arrays for MIMO communications systems. When a MIMO receiver isimplemented within a small portable communications device, there islittle available volume for a large number of antenna elements, andmounting the antenna elements on the outside of the communicationsdevices is a problem for the user.

One approach for providing a more compact antenna array for a MIMOreceiver is disclosed in U.S. Pat. No. 6,870,515. Instead of usingspatially diverse antenna elements, polarization diversity is used.Since closely spaced antenna elements are used, this enables a compactantenna array to be provided for a MIMO receiver.

Even though a more compact antenna array is provided, performance of theMIMO communications system is still based on the number of antennaelements at the receiver being equal to or greater than the number ofantenna elements at the transmitter. For example, the '515 patentdiscloses that the number of receive antenna elements is equal to orgreater than the number of transmit antenna elements.

In addition, if two or more received signals are close together inangular distance, generation of different antenna patterns by the MIMOreceiver may not be adequate to determine differences in the receivedsignals. Even if beam forming is used, making the beam sufficientlynarrow or the bore sight adjustable may not be practical or costeffective. Consequently, there is a need to retain some antenna elementsto differentiate these signals.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to reduce the number of antenna elements at a MIMOreceiver as compared to the number of antenna elements at a MIMOtransmitter while still providing a robust MIMO communications system.

This and other objects, features, and advantages in accordance with thepresent invention are provided by a MIMO communications systemcomprising a transmitter for generating M source signals during a timeinterval, with the M source signals being generated for T timeintervals, and a power level of at least one of the M source signalsbeing different in each time interval. A transmit antenna array isconnected to the transmitter, and may comprise M antenna elements fortransmitting the M source signals.

A receiver is synchronized with the transmitter. A receive antenna arrayis connected to the receiver, and may comprise N antenna elements forreceiving at least N different summations of the M source signals duringeach time interval, with the at least N different summations for eachtime interval being linearly independent for providing at least T*Ndifferent summations for the T time intervals. A signal separationprocessor may be connected to the receiver and forms a mixing matrixcomprising up to the at least T*N different summations of the M sourcesignals. The mixing matrix has a rank equal to at least T*N. The signalseparation processor separates desired source signals from the mixingmatrix.

The at least one M source signal having a different power level in eachtime interval may comprise a plurality of the M source signals havingdifferent power levels in each time interval. Changing the power levelin more than one of the M source signals for each time interval makes iteasier for mixing matrix to be populated for the T time intervals. Thereceive antenna array also advantageously receives the at least Ndifferent summations of the M source signals with N antenna elements,wherein N<M, but T*N≧M so that the rank of the mixing matrix is at leastequal to M.

The rank of the mixing matrix determines how many signals can actuallybe separated. The larger the rank, the more signals can be separated.Consequently, a compact antenna array having N antenna elements, whichis less than the M antenna elements in the transmit array, may be usedby a MIMO receiver while still providing a robust MIMO communicationssystem.

The MIMO communications system may be configured as a MIMO-OFDMcommunications system, with each source signal comprising a plurality ofsub-carriers during the T time intervals. The sub-carriers for eachsource signal may be divided into a plurality of groups, and adjacentgroups of sub-carriers have different power levels. In one embodiment, arespective power envelope for the plurality of sub-carriers for eachsource signal is constant. In another embodiment, the respective powerenvelope for the plurality of sub-carriers for each source signal mayvary in a known order so that the receiver can track groupings of thesub-carriers.

There are a number of different embodiments of the receive antennaarray. The N antenna elements may be correlated for forming a phasedarray. In another embodiment, the N correlated antenna elements maycomprise at least one active antenna element and up to N−1 passiveantenna elements for forming a switched beam antenna. In addition, atleast two of the N correlated antenna elements may have differentpolarizations.

Other embodiments of the receive antenna array may have a multipliereffect on the received M different summations of the M source signals.This advantageously allows the rank of the mixing matrix to be furtherincreased without having to increase the number of N antenna elements inthe receive antenna array. By increasing the rank of the mixing matrix,more signals can be separated by the blind signal separation processor.

The multiplier effect on the number of received M different summationsof the M source signals may be accomplished using one or a combinationof the following. Array deflection involves changing the elevation ofthe antenna patterns for receiving additional summations of the sourcesignals. Path selection may be performed so that all of the summationsof the source signals used to populate the mixing matrix are correlatedand/or statistically independent. Signal splitting may also be used forfurther populating the mixing matrix. The different summation signalsmay be split using spreading codes, or they may be split into in-phase(I) and quadrature (Q) components.

The signal separation processor may be a blind signal separationprocessor. The blind signal separation processor may separate thedesired source signals from the mixing matrix based on at least one ofprincipal component analysis (PCA), independent component analysis(ICA), and single value decomposition (SVD).

Alternatively, the signal separation processor may separate the desiredsource signals from the mixing matrix based on a knowledge basedprocessing signal extraction process. The knowledge based signalseparation process may separates the desired source signals from themixing matrix based on at least one of a zero forcing (ZF) process, anda minimum mean squared estimation (MMSE) process.

Another aspect of the invention is directed to a method for operating aMIMO communications system as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a MIMO communications system in accordancewith the present invention.

FIG. 2 is a more detailed block diagram of the elements on the receiveside of the MIMO communications system as shown in FIG. 1.

FIG. 3 is a block diagram of a MIMO receiver operating based on arraydeflection for providing different summations of signals for blindsignal separation processing in accordance with the present invention.

FIG. 4 is a block diagram of a MIMO receiver operating based on pathselection for providing different summations of signals for blind signalseparation processing in accordance with the present invention.

FIG. 5 is a block diagram of a MIMO receiver operating based onspreading codes for providing additional summations of signals for blindsignal separation processing in accordance with the present invention.

FIG. 6 is a block diagram of a MIMO receiver operating based on in-phaseand quadrature signal components for providing additional summations ofsignals for blind signal separation processing in accordance with thepresent invention.

FIG. 7 is a block diagram of a MIMO transmitter in accordance with thepresent invention.

FIG. 8 is a block diagram of a MIMO receiver in accordance with thepresent invention.

FIG. 9 is a block diagram of a MIMO-OFDM transmitter in accordance withthe present invention.

FIG. 10 is a block diagram of a MIMO-OFDM receiver in accordance withthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

Referring initially to FIG. 1, a MIMO communications system 20 will nowbe described. The MIMO communications system 20 comprises a transmitter30, a transmit antenna array 32, a receiver 40 and a receive antennaarray 42. As readily appreciated by those skilled in the art, thetransmitter 30 and receiver 30 may be replaced with transceivers.Consequently, their respective antenna arrays 32, 42 support two-waydata exchanges. However, for purposes of illustrating the presentinvention, reference will be made to a transmitter 30 and a receiver 40.

The transmit antenna array 32 includes M antenna elements 33(1)-33(M)for transmitting M source signals 34(1)-34(M). The antenna elements33(1)-33(M) may be spatially correlated, for example. The source signals34(1)-34(M) may be generally referred to by reference numeral 34, andthe antenna elements 33(1)-33(M) may be generally referred to byreference numeral 33.

The receive antenna array 42 includes N antenna elements 43(1)-43(N) forreceiving at least M different summations of the M source signals, withN being less than M. Since N<M, a compact antenna array may be used atthe receiver 40 while still obtaining a robust MIMO communicationssystem 20, as will be discussed in greater detail below. The antennaelements 43(1)-43(N) may be generally referred to by reference numeral43.

The respective antenna arrays 32, 42 are used in a multi-path richenvironment such that due to the presence of various scattering objects(buildings, cars, hills, etc.) in the environment, each signalexperiences multipath propagation. Each path may be thought of as adifferent communications channel. Thus, reference numeral 50 in FIG. 1represents a scattering environment resulting in multiple channelsbetween the transmit and receive antenna arrays 32, 42. Data istransmitted from the transmit antenna arrays 32 using a space-timecoding (STC) transmission method as is known in the art.

In addition to the M source signals, L interferer source signals 35 froman interfere 37 may exist within the scattering environment 50 forinterfering with the separation of the desired source signals. Variousmeans to increase the mixing matrix may be advantageously used topopulate the mixing matrix beyond a rank of M, as will be discussed ingreater detail below.

The receive antenna array 42 captures the M different summations of theM source signals 34 and a signal processing technique is then applied toseparate the signals. A blind signal separation (BSS) processor 44 isconnected to the receiver 40 for forming a mixing matrix 46 comprisingthe at least M different summations of the M source signals so that themixing matrix has a rank equal to at least M. The blind signalseparation processor 44 separates desired source signals from the mixingmatrix 46.

As discussed in great detail in U.S. patent application Ser. No.11/232,500, which is incorporated herein by reference in its entirety,three commonly used techniques that fall under blind signal separationare principal component analysis (PCA), independent component analysis(ICA), and singular value decomposition (SVD). As long as the signalsare independent in some measurable characteristic, and if their signalsums are linearly independent from each other, one or more of theseblind signal separation techniques may be used to separate independentor desired source signals from a mixture of the source signals. Themeasurable characteristic is often some combination of the first,second, third or fourth moments of the signals.

PCA whitens the signals, uses first and second moments, and rotates thedata set based on correlation properties. If the signal-to-noise ratiosof the source signals are high, the signal separation process can stopwith PCA.

If the signal-to-noise ratios of the source signals are low, then ICAseparates the source signals based on statistical attributes involvingthe third and fourth moments of the source signals. Some source signalsare Gaussian, and their third and fourth moments are dependent on thefirst and second moments. A random noise source can be Gaussian, andspread spectrum signals are designed to appear Gaussian to decoders byother than their specific spreading code. Under specific conditions, anaggregate of signals can appear Gaussian due the central limit theorem.The ICA approach can separate one Gaussian signal. As an alternative toICA and PCA, SVD separates source signals from the mixture of sourcesignals based upon their eigenvalues.

As an alternative to a blind signal separation processor, a signalseparation processor may be used for separating the desired sourcesignals from the mixing matrix based on a knowledge based processingsignal extraction process. The knowledge based signal separation processseparates the desired source signals from the mixing matrix based on atleast one of a zero forcing (ZF) process, and a minimum mean squaredestimation (MMSE) process, for example.

The different elements on the receive side of the MIMO communicationssystem 20 will now be discussed in greater detail with reference to FIG.2. The receive antenna array 42 includes N antenna elements 43(1)-43(N)for receiving at least M different summations of the M source signals34, with N and M being greater than 1 and with N being less than M. Thereceive antenna array 42 is not limited to any particular configuration.The receive antenna array 42 may include one or more antenna elements43. The antenna elements 43 may be configured so that the antenna arrayforms a phased array or switched beam antenna, for example.

For the purpose of building the mixing matrix 46, the goal is createdifferent sums of signals. The signals of interest can actually alwaysbe lower than the interferers in this application and still beseparated. Because of this significant difference in purpose, thedistances between antenna elements 43 need not be of a specificseparation as is normally required by active and passive beam formingantenna arrays.

The receiver 40 is connected downstream to the receive antenna array 42for receiving at least M different summations of the M source signals34. A blind signal separation processor 44 is downstream to the receiver40. Even though the processor 44 is illustrated separate form thereceiver 40, the processor may also be included within the receiver. Thedifferent summations of the M source signals 34 received by the receiver40 are used to populate the mixing matrix 46. The mixing matrix 46 isthen processed by one or more blind signal separation processing modules62, 64 and 66 within the processor 60.

The blind signal separation processing modules include a PCA module 62,an ICA module 64 and an SVD module 66. These modules 62, 64 and 66 maybe configured as part of the blind signal separation processor 44. ThePCA module 62 operates based on the first and second moments of thedifferent summations of the received source signals, whereas the ICAmodule 64 operates based on the third and fourth moments of the samesignals. The SVD module 66 performs signal separation based on theeigenvalues of the different summations of the received source signals.

The correlation processing initially performed by the PCA module 62determines an initial separation matrix 68(1) for the differentsummations of the source signals, and the ICA module 64 then determinesan enhanced separation matrix 68(2) for separating the source signals inthe mixing matrix 46. If the signals are separated by the SVD module 66,a separation matrix 68(3) is also determined for separating thedifferent summations of the received source signals in the mixing matrix46.

From each respective separation matrix 68(1)-68(3), the separatedsignals are represented by reference number 49. The separated signals 49then undergo signal analysis by a signal analysis module 70 to determinewhich signals are of interest and which signals are interferers. Anapplication dependent processing module 72 processes the signals outputfrom the signal analysis module 70.

The decision on which signals are of interest may not always involve thefinal signal to be decoded. For instance, the application may call foridentifying interferers and subtracting them from the differentsummations of the received source signals, and then feeding the reducedsignal to a waveform decoder. In this case, the signals of interest arethe ones that ultimately end up being rejected.

The rank of the mixing matrix 46 determines how many signals canactually be separated. For example, a mixing matrix having a rank of 4means that 4 source signals can be separated. Ideally, the rank of themixing matrix 46 should at least be equal to the number of signalsources M. The larger the rank, the more signals that can be separated.As the number of sources M increases, then so does the required numberof antenna elements N. The '515 patent discussed in the backgroundsection discloses that the number of antenna elements N at the receiverare equal to or greater than the number of antenna elements M at thetransmitter, i.e., N≧M.

The receive antenna array 42 advantageously receives the M differentsummations of the M source signals 34 with N antenna elements 33,wherein N<M. The N antenna elements 43 generate at least M differentantenna patterns for receiving the M different summations of the Msource signals. The M different summations of the M source signals 34received by the N antenna elements 43 at the receive antenna array 42are used to populate the mixing matrix 46 so that the mixing matrix hasa rank equal to at least M.

As noted above, the rank of the mixing matrix 46 determines how manysignals can actually be separated. The larger the rank, the more signalscan be separated. Consequently, a compact receive antenna array 42having N antenna elements 43, which is less than the M antenna elements33 in the transmit antenna array 32, may be used by a MIMO receiver 40while still providing a robust MIMO communications system 20.

While M linearly independent summations are the minimum necessary tosupport a full MIMO implementation of M transmit antenna elements 34,there are advantages to exceeding M. For instance, not all of the Nantenna elements 43 at the receive antenna array 42 may be oriented toreceive the M linearly independent summations. Likewise, not all of thereceived summations are sufficiently linearly independent. There mayalso be L other signals that degrade the signal to noise ratio inaddition to the M known signal streams being separated.

Consequently, it is advantageous to take advantage of increasing therank of the mixing matrix to M+L when possible. Another advantage ofseparating interference or noise sources is a resultant reduction in thesignal-to-noise ratio, which allows higher data rates, lower errorrates, and/or decreased transmission power.

For example, L interferer source signals 35 may exist for interferingwith separation of the desired source signals 34 from the mixing matrix,with L being greater than 1. If increasing the rank of the mixing matrixhas been exhausted without having to add additional antenna elements,then adding at least one additional antenna element will provideadditional means to increase the rank of the mixing matrix. Addingadditional elements may still leave the count of elements below M of theclassical MIMO approach, or it may return the number of elements to M,or even increase it beyond M. Depending on the gains achieved byincreasing the mixing matrix rank, it may still be worthwhile to do so,even though it increases the receiver antenna element count. Forexample, a mixing matrix of rank M+L requiring M elements will often bea superior implementation versus an M element implementation usingclassical processing MIMO receiver. However, for purposes ofillustrating the present invention, the following discussion will focuson the M source signals.

There are a number of different embodiments of the receive antenna array42. The N antenna elements 43 may be correlated for forming a phasedarray. In another embodiment, the N correlated antenna elements 43 maycomprise at least one active antenna element and up to N−1 passiveantenna elements for forming a switched beam antenna. In addition, atleast two of the N correlated antenna elements may have differentpolarizations.

Other embodiments of the receive antenna array 42 have a multipliereffect on the received M different summations of the M source signals.This advantageously allows the rank of the mixing matrix 46 to befurther increased without having to increase the number of N antennaelements 43 in the receive antenna array 42. By increasing the rank ofthe mixing matrix 46, more signals can be separated by the blind signalseparation processor 44.

The multiplier effect on the number of received M different summationsof the M source signals 34 may be accomplished using one or acombination of the following. Array deflection involves changing theelevation of the antenna patterns for receiving additional summations ofthe source signals 34. Path selection may be performed so that all ofthe summations of the source signals 34 used to populate the mixingmatrix 46 are correlated and/or statistically independent. Signalsplitting may also be used for further populating the mixing matrix 46.The different summation signals may be split using spreading codes, orthey may be split into in-phase (I) and quadrature (Q) components.

The different embodiments of the receive antenna array will now bediscussed in greater detail with reference to FIGS. 3-6. Referring nowto FIG. 3, array deflection will be discussed. The receive antenna array142 comprises N antenna elements 143 for generating N initial antennapatterns for receiving N different summations of the M source signals.The receive antenna array 142 also comprises an elevation controller 141for selectively changing an elevation of at least one of the N initialantenna patterns for generating at least one additional antenna patternso that at least one additional different summation of the M sourcesignals is received thereby.

A receiver 140 is connected to the receive antenna array 142 andreceives the N different summations of the M source signals using the Ninitial antenna patterns, and also receives the at least one additionaldifferent summation of the M source signals using the at least oneadditional antenna pattern.

A blind signal separation processor 144 is connected to the receiver 140for forming a mixing matrix 146 comprising the N different summations ofthe M source signals and the at least one additional different summationof the M source signals. The mixing matrix has a rank equal to N plusthe number of additional different summations of the M source signalsreceived using the additional antenna patterns. A resulting rank of themixing matrix 146 is at least equal to M. The processor 144 separatesdesired signals from the mixing matrix 146.

In general, any antenna array means which provides signal sums suitablefor increasing the rank of the mixing matrix can be utilized with adeflection mechanism. The deflection will generate two distinct andmixing matrix usable signal sums for each of the antenna array means.There is therefore a 2 times multiplier effect by utilization of thistechnique.

If the array deflection is segmented into K distinct areas associatedwith an antenna, each of the K areas can provide for 2 independentdeflection areas and entries into the mixing matrix. For instance, ifthe antenna array can provide N summations by itself and there are Kdistinct deflection areas, the number of signal sums in the mixingmatrix may be 2NK.

Separating source signals provided by M signal sources based on pathselection will be discussed in reference to FIG. 4. The receive antennaarray 242 comprising N elements 243 for forming at least N antenna beamsfor receiving at least N different summations of the M source signals,with N and M being greater than 2.

A controller 250 is connected to the antenna array 242 for selectivelyforming the at least N antenna beams. A receiver assembly 240 isconnected to the antenna array 242 for receiving the at least Ndifferent summations of the M source signals. A blind signal separationprocessor 244 is connected to the receiver assembly 240 for forming amixing matrix 246 comprising up to the at least N different summationsof the M source signals.

The blind signal separation processor 244 also determines if thedifferent summations of the M source signals are correlated orstatistically independent, and if not, then cooperates with thecontroller 250 for forming different beams for receiving new differentsummations of the M source signals to replace the different summationsof the M source signals that are not correlated or statisticallyindependent in the mixing matrix 246. As a result, at least M differentsummations of the source signals are received so that the mixing matrixhas a rank at least equal to M. The desired source signals are thenseparated from the mixing matrix 246.

A rake receiver is a radio receiver designed to counter the effects ofmultipath fading. It does this by using several independent receiverseach delayed slightly in order to tune in to the individual multipathcomponents. It can be used by most types of radio access networks. Ithas been found to be especially beneficial for spreading code types ofmodulation. Its ability to select specific incident signal paths make itsuitable as a means to change the paths fed to the blind signalseparation processor 244.

Selectively forming the N antenna beams as discussed above may beapplied to all radio access networks, as readily understood by thoseskilled in the art. For CDMA systems, the receiver assembly 240comprises N rake receivers 256. Each rake receiver 256 comprises kfingers for selecting k different multipath components for each one ofthe N different summations of the M source signals received by therespective antenna element connected thereto. In this configuration, theblind signal separation processor 244 is connected to the N rakereceivers 256 for forming the mixing matrix 246. The mixing matrix 246comprises up to at least kN different multipath components of the atleast N different summations of the M source signals, and the mixingmatrix has a rank equal up to kN, where kN is at least equal to M.

In particular, when CDMA waveforms propagate they often encountermultiple paths from the source to the destination. A rake receiver 256is specifically designed to capture a number of these individualinstances and combine them for a more robust signal decoding. While theoriginal signal propagates along each path, its properties are modifiedby the unique characteristics of the path. In some circumstances, themodification to the correlation and/or statistical properties of thereceived signal will be large enough so that they can be treated asseparable signal streams. A modified rake receiver 256 could be used toextract each modified stream and feed it as a unique entry into themixing matrix 246. While this means of increasing the rank will notalways be available, it will tend to be available in high multipathenvironments when it is most likely needed.

While a rake receiver 256 can exploit the different paths, the moregeneral approach applicable to any modulation technique is beam forming.This differs from the rake receiver 256 since beam forming is used fordesired signal enhancement as well as desired signal rejection. Thedifference however is that the rejected signal may actually be anotherversion of the signal intended for the receiver. However, the receiverassembly 240 needs to detect a number of these unique propagation pathversions of the same signal in order to build the mixing matrix 246 to asufficient rank.

Signal splitting is also used for further populating the mixing matrixA. In one approach, the summation signals are split using spreadingcodes. In another approach, the summation signals are split usingin-phase (I) and quadrature (Q) modules.

Signal splitting using spreading codes will now be discussed inreference to FIG. 5. The receive antenna array 342 comprising N antennaelements 343 for receiving at least N different summations of the Msource signals. A code despreader 350 is connected to the N antennaelements 343 for decoding the at least N different summations of the Msource signals. Each one of the N different summations includes k codesfor providing k different summations of the M source signals associatedtherewith.

A receiver assembly 340 is connected to the code despreader 350 forreceiving at least kN different summations of the M source signals. Ablind signal separation processor 344 is connected to the receiverassembly 340 for forming a mixing matrix 346 comprising the at least kNdifferent summations of the M source signals. The mixing matrix 346 hasa rank equal up to kN, with a resulting rank at least being equal to M.The blind signal separation processor 344 separates desired sourcesignals from the mixing matrix 346.

Depending on the modulation of the received signals, the above describedsignal splitting may be used for increasing the rank of the mixingmatrix without increasing the number N of antenna elements. CDMA IS-95,CDMA2000 and WCDMA are examples of spread spectrum communicationssystems in which spreading codes are used. A common thread is that aunique code is processed with each signal to spread the data over alarger frequency band.

The same spreading code is processed with the received signal sum(desired signal, undesired signals and unknown noise sources). Thiscauses the desired signal to be reconstructed back to its originalfrequency bandwidth, while the interferers are spread over the widefrequency band.

The above listed CDMA implementations actually have many signal streamssimultaneously using the same frequency band. Each signal stream uses acode that is ideally orthogonal to all the others. If this condition ismet at the decoder, it means that only the signal of interest will bedespread.

There often is some correlation between the CDMA signals, so theinterfering signals are somewhat reconstructed along with the desiredsignal. This is often due to the delay experienced by the individualsignals, and also the multipath occurrences of the signals. Some of theundesired signals, especially the CDMA ones, will increase in value. Theincrease will not be as significant as for the desired signal, but itwill still increase the overall noise value, and therefore decrease thesignal-to-noise ratio.

The form of the despread signals equation and the signals themselvesmeet the criteria for blind signal separation processing. In fact, ifone of the dispreading codes is individually applied for each knownsignal received by the receiver assembly 340, individual summations thatmeet the ICA model requirements are obtained.

Therefore, there are as many row entries available for the mixing matrixas known codes, assuming of course, that they each produce linearlyindependent significant value. Under the right circumstances this willallow an increase of the mixing matrix to a value greater than thenumber of codes. For example, N antenna elements and M codes may provideNM matrix rows.

For illustrative purposes, 3 codes are assumed known and the 3 knowncode signals retain their orthogonality. In the code despreader 350, themixing matrix A has top 3 rows and bottom 3 rows each due to an antennastream after each stream has been despread by the 3 known codes. The offdiagonal 0 values are due to the orthogonality of the codes. The columnentries 4, 5 and 6 are for the general case of unknown signals of thesame index.

$\begin{bmatrix}x_{1} \\x_{2} \\x_{3} \\x_{4} \\x_{5} \\x_{6}\end{bmatrix} = {\begin{bmatrix}a_{11} & 0 & 0 & a_{14} & a_{15} & a_{16} \\0 & a_{22} & 0 & a_{24} & a_{25} & a_{26} \\0 & 0 & a_{33} & a_{34} & a_{35} & a_{36} \\a_{41} & 0 & 0 & a_{44} & a_{45} & a_{46} \\0 & a_{52} & 0 & a_{54} & a_{55} & a_{56} \\0 & 0 & a_{63} & a_{64} & a_{56} & a_{66}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\s_{3} \\s_{4} \\s_{5} \\s_{6}\end{bmatrix}}$

The signals corresponding to the column entries 4, 5 and 6 can be otherpath versions of the known codes, or other cell signals of unknowncodes. Also, one signal may be Gaussian and the other signal is eitherCDMA signal groups obeying the central limit theorem so that they appearas a single Gaussian signal, e.g., release 4 channels. In other words, asufficient amount of non-random signals will add up to a Gaussiansignal. The interferers may be non-Gaussian signal sources or at mostone Gaussian signal unknown to the network.

After despreading the known codes in the code despreader 350, the blindsignal separation processor 344 receives a mixing matrix 346 of rank 6.The rank of 6 is derived based upon 2 antenna elements multiplied by afactor of 3 since 3 codes are known.

The 6 signals are applied to the blind signal separation processor 344wherein the mixing matrix 346 having a rank of 6 is formed. The blindsignal separation processor 344 determines the separation matrix W fromonly the received signals modified by the channels: x=As, where A is themixing matrix. In the illustrated example, 6 signals are separable.

The blind signal separation processor 344 selects the signals to bedecoded. For example, the interferer signals may be dropped and allversions of the desired signals are selected. The selected signals areapplied to a demodulator module for demodulation. The demodulator useswell known equalization techniques that combine the multipath versionsof the same signal.

In the more general case the off diagonal values are shown as 0 abovefor simplicity, could actually be nonzero. This would be the more usualcase when the correlation properties between the coded signals are notperfect. This would represent additional noise to each separated signal.However, as previously shown the rank of the matrix is sufficient toseparate these signals, so their value will be significantly reducedafter the blind signal separation processing. This leads to a decreasein noise, an increase in signal to noise ratio, and as indicated byShannon's law an increase in channel capacity.

Referring now to FIG. 6, the other approach for increasing the rank ofthe mixing matrix A without increasing the number N of antenna elementsis to separate a received mixed signal into its in-phase (I) andquadrature (Q) components. I and Q components of a coherent RF signalare components whose amplitudes are the same but whose phases areseparated by 90 degrees.

The receive antenna array 442 comprising N antenna elements 443 forreceiving at least N different summations of the M source signals. Arespective in-phase and quadrature module 450 is downstream to eachantenna element 443 for separating each one of the N differentsummations of the M source signals received thereby into an in-phase andquadrature component set.

A receiver assembly 440 is downstream to each in-phase and quadraturemodule 450 for receiving the at least N in-phase and quadraturecomponent sets for the at least N different summations of the M sourcesignals. A blind signal separation processor 444 is downstream to thereceiver assembly 440 for forming a mixing matrix 446 comprising atleast 2N different summations of the M source signals. Each in-phase andquadrature component set provides 2 inputs into the mixing matrix 446.The mixing matrix 446 has a rank equal up to 2N, and the blind signalseparation processor 444 separates desired source signals 514 from themixing matrix 512.

By separating the received mixed signals into I and Q components, thesize of the mixing matrix increases by a factor of 2. As long as the Iand Q components are encoded with different data streams, then the mixedsignal received at any antenna element may be split into two differentmixed signals.

In the case of differential encoding the nature of the modulation needsto be analyzed to determine if I and Q meet the linearity requirement.For instance, it has been shown for GSM that the GMSK encoding can beassumed linear when used with appropriate filtering, and processed inthe receiver as if it were BPSK encoding. Since BPSK meets therequirements for blind signal separation processing, the I and Q processdescribed can be used.

I and Q components can be used with any of the above described antennaarray embodiments to populate the mixing matrix A. When I and Q is used,the mixing matrix A can be populated as if 2 times the number of antennaelements were used. The antenna elements could be of any diversity formsuch as uncorrelated, correlated or polarized. The N antenna elementswith each element's signal sum split into I and Q components providesfor 2N independent mixed signal sums. As a result, the rank of themixing matrix is 2N, where 2N is at least equal to or greater than M.This mechanism could also be used with the antenna array deflectiontechnique to create more sums of signals. Each of these sums could inturn also be separated into I and Q components. A factor of 2 from I andQ, N antenna elements, and K deflections areas for the antenna arraywould provide 2KN sums for the mixing matrix.

Referring now to FIGS. 7 and 8, another aspect of the present inventionis directed to a MIMO communications system wherein a MIMO transmitter500 changes the power levels for each layered space data stream on atime slotted basis. The data streams therefore arrive at the MIMOreceiver 502 with various power levels, which provides suitabledifferences in the received signals for population of the mixing matrixfor signal separation processing.

A data stream for transmission is received by a sub-channel creationblock 510. The sub-channel creation block 510 breaks the data streaminto parallel data streams for channels 1 through M.

Each of the parallel data streams goes through an RF chain to an antennaelement 523(1)-523(M) associated therewith. Each RF chain includes adigital-to-analog converter 512(1)-512(M) for converting the data streamto a base band. The base band signal is then multiplied by its carrierfrequency in the RF modulation block 514(1)-514(M) to obtain the desiredfrequency for transmission. After modulation, the data stream isamplified by a power amplifier 516(1)-516(M) before being transmitted bythe antenna element 523(1)-523(M) associated therewith. This isindividually done for each one of the parallel data streams 1 through M.

The power level of the individual data streams in each RF chain ischanged by a time interval gain control block 520. The power level orgain of each parallel data stream may be changed in thedigital-to-analog converters 512(1)-512(M) or in the RF modulationblocks 514(1)-514(M). This change is synchronized with the sub-channelcreation block 510. Collectively, the M data streams are transmitted bythe M antenna elements 523(1)-523(M).

As a minimum, the power level in one of the M data streams needs to bechanged. When changed, the MIMO receiver 502 will receive a linearlyindependent sum during a respective time interval. However, it ispreferable to vary the power level in several of the data streams (up toM) to insure a more robust MIMO receiver 502. Even though the powerlevel of several of the data streams is being changed, the MIMO receiver502 still receives one linearly independent summation signal for therespective time interval.

On the receive side, the signals are received by the antenna elements543(1)-543(N). Up to N summation signals are received by the antennaelements 543(1)-543(N). As noted above, for each time interval, M sourcesignals are transmitted by the MIMO transmitter 500. There is noone-to-one relationship between the signals transmitted from the Mtransmit antenna elements 523(1)-523(M) and the signals received by theN receive antenna elements 543(1)-543(N). As is typically the case, eachreceive antenna element 543(1)-543(N) receives more than one of thetransmit signals and multiple incidences of the same transmit signals.The ideal orthogonality of the channels is very unlikely in a realcommunications environment.

From each receive antenna element 543(1)-543(N) is an RF chain. Areceive amplifier 522(1)-522(N) amplifies the received data stream. AnRF demodulation block 524(1)-524(N) multiplies received data stream byits carrier frequency and filters out the extraneous harmonics to obtainthe symbols at the base band signal. Once the base band signal isobtained, it passes through an analog-to-digital converter 526(1)-526(N)to a mixing matrix module 530.

The mixing matrix module 530 populates the mixing matrix to be used forseparating the desired signals. The mixing matrix module 530 interfaceswith a time interval identification module 532 so that the mixing matrixcreated for each time interval matches up with the corresponding timeintervals in the MIMO transmitter 500.

The output of the mixing matrix module 530 is applied to a spatialde-multiplexer module 534. Separation of the desired signals from themixing matrix is performed in this module. The signal separation may bebased upon a knowledge based signal extraction process or a blind signalseparation, or a combination of both. The bind signal separation processand the knowledge based signal extraction process has been describedabove. The separated signals are then applied to a sub-channel combiner536 for providing the received data stream for decoding.

In summary, the transmitter 500 generates M source signals during a timeinterval, with the M source signals being generated for T timeintervals, and a power level of at least one of the M source signals isdifferent in each time interval. A transmit antenna array is connectedto the transmitter 500, and comprising M antenna elements 523(1)-523(M)for transmitting the M source signals.

The receiver 502 is synchronized with the transmitter 500. A receiveantenna array is connected to the receiver 502, and comprising N antennaelements 543(1)-543(N) for receiving at least N different summations ofthe M source signals during each time interval, with the at least Ndifferent summations for each time interval being linearly independentfor providing at least T*N different summations for the T timeintervals. The mixing matrix module 530 may be implemented in a signalprocessor, for example, and forms a mixing matrix comprising up to theat least T*N different summations of the M source signals. The mixingmatrix has a rank equal to at least T*N. The signal separation processorseparates desired source signals from the mixing matrix.

Changing the power levels for each layered space data stream may also beapplied to a MIMO-OFDM communications system, as illustrated in FIGS. 9and 10, which includes a MIMO-OFDM transmitter 600 and a MIMO-OFDMreceiver 602. OFDM stands for orthogonal frequency divisionmultiplexing. An OFDM system effectively partitions the operatingfrequency band into a number of frequency sub-channels (or frequencybins), each of which is associated with a respective sub-carrier onwhich data may be modulated. Typically, the data to be transmitted isencoded with a particular coding scheme to generate coded bits.

In particular, each source signal transmitted by the MIMO-OFDMtransmitter 600 comprising a plurality of sub-carriers during the T timeintervals. The sub-carriers for each source signal may be divided into aplurality of groups, and adjacent groups of sub-carriers have differentpower levels. Alternatively, a respective power envelope for theplurality of sub-carriers for each source signal may be constant. Arespective power envelope for the plurality of sub-carriers for eachsource signal may vary in a known order so that the MIMO-OFDM receiver602 can track groupings of the sub-carriers.

The illustrated MIMO-OFDM system is similar to the illustrated MIMOsystem illustrated in FIGS. 7-8 except it now includes an inverse fastFourier transform (IFFT) block 608(1)-608(M) in the MIMO-OFDMtransmitter 600 and a fast Fourier transform (FFT) block 625(1)-625(M)in the MIMO-OFDM receiver 602. With the IFFT blocks 608(1)-608(M), eachRF chain is now transmitting a group of sub-channels instead of a singlechannel in the MIMO communications system.

The IFFT blocks 608(1)-608(M) take the data streams that are parallel inthe frequency domain and provides their aggregate single timerepresentation to the digital-to-analog converters 612(1)-612(M). Thesub-channel creation block 610 determines which sub-channels will besent to which antenna element.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

1. A multiple-input multiple-output (MIMO) communications systemcomprising: a transmitter for generating M source signals during a timeinterval, with the M source signals being generated for T timeintervals, and a power level of at least one of the M source signalsbeing different in each time interval; a transmit antenna arrayconnected to said transmitter, and comprising M antenna elements fortransmitting the M source signals; a receiver synchronized with saidtransmitter; a receive antenna array connected to said receiver, andcomprising N antenna elements for receiving at least N differentsummations of the M source signals during each time interval, with theat least N different summations for each time interval being linearlyindependent for providing at least T*N different summations for the Ttime intervals; and a signal separation processor connected to saidreceiver and forming a mixing matrix comprising up to the at least T*Ndifferent summations of the M source signals, the mixing matrix having arank equal to at least T*N, said signal separation processor forseparating desired source signals from the mixing matrix.
 2. A MIMOcommunications system according to claim 1 wherein the at least one Msource signal having a different power level in each time intervalcomprises a plurality of the M source signals having different powerlevels in each time interval.
 3. A MIMO communications system accordingto claim 1 wherein N<M and T*N≧M so that the rank of the mixing matrixis at least equal to M.
 4. A MIMO communications system according toclaim 3 wherein N=1 and T=M.
 5. A MIMO communications system accordingto claim 1 wherein N≧M and T>M so that the rank of the mixing matrix isgreater than M for enabling interference signals as well as the M sourcesignals to be separated.
 6. A MIMO communications system according toclaim 1 wherein the MIMO communications system is configured as aMIMO-OFDM communications system, with each source signal comprising aplurality of sub-carriers during the T time intervals.
 7. A MIMOcommunications system according to claim 6 wherein the sub-carriers foreach source signal are divided into a plurality of groups, and adjacentgroups of sub-carriers have different power levels.
 8. A MIMOcommunications system according to claim 6 wherein a respective powerenvelope for the plurality of sub-carriers for each source signal isconstant.
 9. A MIMO communications system according to claim 6 wherein arespective power envelope for the plurality of sub-carriers for eachsource signal varies in a known order so that said receiver can trackgroupings of the sub-carriers.
 10. A MIMO communications systemaccording to claim 1 wherein said receive antenna array comprises Ncorrelated antenna elements for forming a phased array.
 11. A MIMOcommunications system according to claim 1 wherein said receive antennaarray comprises N correlated antenna elements, said N correlated antennaelements comprising at least one active antenna element and up to N−1passive antenna elements for forming a switched beam antenna.
 12. A MIMOcommunications system according to claim 1 wherein said receive antennaarray comprises N correlated antenna elements, and wherein at least twoof said N correlated antenna elements have different polarizations. 13.A MIMO communications system according to claim 12 wherein the differentpolarizations are orthogonal to one another.
 14. A MIMO communicationssystem according to claim 1 wherein said receive antenna array generatesN initial antenna patterns during each time interval for receiving theat least N different summations of the M source signals; and furthercomprising an elevation controller connected to said receive antennaarray for selectively changing an elevation of at least one of the Ninitial antenna patterns so that at least one additional differentantenna pattern is generated during each time interval for receiving atleast one additional summation of the M source signals; and wherein themixing matrix further comprises the at least one additional differentsummation of the M source signals for each time interval, the mixingmatrix having a rank equal to T*N plus the number of additionaldifferent summations of the M source signals received during the T timeintervals using the additional antenna patterns.
 15. A MIMOcommunications system according to claim 1 wherein said receive antennaarray generates at least N antenna beams during each time interval forreceiving the at least N different summations of the M source signals,with N and M being greater than 2; and further comprising a controllerconnected to said receive antenna array for selectively forming the atleast N antenna beams; said signal separation processor also determiningif the at least N different summations of the M source signals duringeach time interval are uncorrelated or statistically independent, and ifnot, then cooperating with said controller for forming different beamsfor receiving new different summations of the M source signals toreplace the different summations of the M source signals that arecorrelated or statistically dependent in the mixing, with ones that areuncorrelated or statistically independent.
 16. A MIMO communicationssystem according to claim 1 further comprising a code despreaderconnected to said N antenna elements for decoding the at least Ndifferent summations of the M source signals during each time interval,each one of the at least N different summations including k codes forproviding at least k different summations; wherein said receiver isconnected to said despreader for receiving up to k*(T*N) summations ofthe M source signals for the T time intervals; and wherein said signalseparation processor forms the mixing matrix comprising the at leastk*(T*N) different summations of the M source signals.
 17. A MIMOcommunications system according to claim 1 further comprising arespective in-phase and quadrature module connected downstream to eachantenna element in said receive antenna array for separating each one ofthe N different summations of the M source signals received thereby intoan in-phase and quadrature component set during each time interval; andwherein said signal separation processor forms the mixing matrixcomprising at least 2(T*N) different summations of the M source signalsfor the T time intervals, with each in-phase and quadrature componentset providing 2 inputs into the mixing matrix, with a resulting rank atleast being equal 2(T*N).
 18. A MIMO communications system according toclaim 1 wherein said signal separation processor comprises a blindsignal separation processor.
 19. A MIMO communications system accordingto claim 18 wherein said blind signal separation processor separates thedesired source signals from the mixing matrix based on at least one ofprincipal component analysis (PCA), independent component analysis(ICA), and single value decomposition (SVD).
 20. A MIMO communicationssystem according to claim 1 wherein said signal separation processorseparates the desired source signals from the mixing matrix based on aknowledge based processing signal extraction process.
 21. A MIMOcommunications system according to claim 20 wherein the knowledge basedsignal separation processor separates the desired source signals fromthe mixing matrix based on at least one of a zero forcing (ZF) process,and a signals from the mixing matrix based on a minimum mean squaredestimation (MMSE) process.
 22. A MIMO communications system according toclaim 1 wherein said signal separation processor separates the desiredsource signals from the mixing matrix based on a combination of aknowledge based signal extraction process and a blind signal separationprocess.
 23. A method for operating a multiple-input multiple-output(MIMO) communications system comprising: generating M source signalsduring a time interval, with the M source signals being generated for Ttime intervals, and a power level of at least one of the M sourcesignals being different in each time interval; transmitting the M sourcesignals using a transmit antenna array comprising M antenna elements;receiving at least N different summations of the M source signals duringeach time interval using a receive antenna array and comprising Nantenna elements, with the at least N different summations for each timeinterval being linearly independent for providing at least T*N differentsummations for the T time intervals; forming a mixing matrix comprisingup to the at least T*N different summations of the M source signals, themixing matrix having a rank equal to at least T*N; and separatingdesired source signals from the mixing matrix.
 24. A method according toclaim 23 wherein the at least one M source signal having a differentpower level in each time interval comprises a plurality of the M sourcesignals having different power levels in each time interval.
 25. Amethod according to claim 23 wherein N<M and T*N≧M so that the rank ofthe mixing matrix is at least equal to M.
 26. A method according toclaim 23 wherein N≧M and T>M so that the rank of the mixing matrix isgreater than M for enabling interference signals as well as the M sourcesignals to be separated.
 27. A method according to claim 23 wherein theMIMO communications system is configured as a MIMO-OFDM communicationssystem, with each source signal comprising a plurality of sub-carriersduring the T time intervals.
 28. A method according to claim 27 whereinthe sub-carriers for each source signal are divided into a plurality ofgroups, and adjacent groups of sub-carriers have different power levels.29. A method according to claim 23 wherein the receive antenna arraycomprises N correlated antenna elements for forming a phased array. 30.A method according to claim 23 wherein the receive antenna arraycomprises N correlated antenna elements, the N correlated antennaelements comprising at least one active antenna element and up to N−1passive antenna elements for forming a switched beam antenna.
 31. Amethod according to claim 23 wherein separating the desired sourcesignals from the mixing matrix is based on at least one of a knowledgebased signal extraction process and a blind signal separation process.